(1) Field of the Invention
The present invention generally relates to an image forming apparatus, such as a printer and a copy machine, and more particularly to an image forming apparatus in which a gradation pattern having a plurality of gradation levels is output by an image output unit to a recording medium and an image signal conversion table used to convert input image signals into output image signals to be supplied to the image output unit is corrected based on reading data obtained by optically reading the gradation pattern and target data for the reading data.
(2) Description of the Related Art
In a conventional image forming apparatus such as a digital copy machine, an image signal conversion table has been used to change the characteristics of output image data with respect to image data and to emphasize specific density parts of an image. The image signal conversion table is generally provided in an image processing unit of the image forming apparatus. The input image signals input by an image reading unit are converted into the output signals to be supplied to an image output unit (for example, having a laser diode) using an image signal conversion table.
Such an image signal conversion table reflects output characteristics of the image forming apparatus. In a production process of the image forming apparatus, the image signal conversion table is set in the image forming apparatus. However, the output characteristics of the image forming apparatus are changed with time. Thus, after the output characteristics of the image forming apparatus are changed, the input image data cannot be fitly converted into the output image data using the image signal conversion table set in the production process of the image forming apparatus. The quality of an image formed by the image forming apparatus deteriorates.
To prevent the quality of an image from deteriorating caused by changing the output characteristics of the image forming apparatus with time, various measures have been considered. Some of them are as follows.
A first method by which an image forming process (e.g., an electrophotographic image forming process) is controlled to prevent the quality of an image from deteriorating has been proposed. In this method, an image pattern having a plurality of density levels is formed on an image holding body such as a photo sensitive body or a transferring body. The reflection light from the image pattern or transmission light through the image pattern is then detected by an optical sensor. Based on the detecting result, a charging potential, a developing bias and an amount of exposure light are controlled.
A second method by which the image signal conversion table as described above is corrected to prevent the quality of an image from deteriorating has been proposed (Japanese Laid Open Patent Application No.5-114962). In this method, a specific image pattern is formed on a recording sheet. After this, the image on the recording sheet is optically read by an image reading unit. Then, based on information obtained by the image reading unit, the image signal conversion table is corrected.
According to the first method described above, the quality of the image can be automatically controlled without handling of an operator. However, the sensitivity of the optical sensor is generally low for a high density portion in which a large amount of toner is provided. Thus, in only an area other than the high density portions, the quality of the image is effectively corrected. In addition, the deterioration of the quality of an image caused by the deterioration with time of a transferring unit and/or a fixing unit can not be improved.
According to the second method described above, although the recording sheet has to be set in the image reading unit by hand, the quality of all portions of an image can be corrected. In addition, the deterioration of the quality of an image caused by the deterioration with time of the transferring unit and/or the fixing unit can be improved.
The concept in which the quality of an image is corrected by the correction of the image signal conversion table can compare to a feedback control process as shown in FIG. 1. The characteristics of the image forming unit deteriorates with time, so that the output data of the image forming unit differs from target data which has been provided. In this case, as shown in FIG. 29, to decrease the difference between the output data of the image forming unit and the target data, the image signal conversion table is corrected.
However, in the above conventional case where the image signal conversion table is corrected, the target data is fixed. Thus, if the target data is unsuitable, the image signal conversion table can not be suitably corrected. For example, the target data is made based on the characteristics of the image forming apparatus in the production process of the image forming apparatus. The target data is then stored in a storage unit, such as a ROM or a RAM, of the image forming apparatus. The target data should be suitable for every image forming apparatus. However, the characteristics of the image forming apparatus are varied due to the variation of components of the image forming apparatus, so that the fixed target data is unsuitable for some of image forming apparatuses. Thus, in some of the image forming apparatuses, the desired quality of an image is not obtained. In addition, since the target data is decided at the production side of the image forming apparatus, an image formed by using the image signal conversion table which is corrected based on the target data do not always satisfy users of the image forming apparatus.